Use of alkali photosensitive compounds to lower work function has been fundamental to the continued use and development of thermionic dispenser cathodes common to most vacuum electronic devices as well as most practical photocathodes. In the notional thermionic dispenser cathode, an alkali-oxide compound impregnates an electrically and thermally conductive refractory matrix, such as sintered tungsten, which is then heated to greater than 1000° C. The heating process reduces the alkali oxide, freeing alkali atoms to diffuse to an emitting surface (due to concentration gradient) where the alkali atoms lower the work function and enable thermionic emission. These cathodes are relatively rugged and exhibit relatively long lifetimes, on the order of tens to hundreds of kilo-hours, but cannot be rapidly gated for use in high frequency applications that require both a high-quality electron (i.e., high frequency electron injectors and high frequency RF sources). Some efforts have been made to evaluate the possible use of commercial thermionic cathodes as photoemitters: temperature was lowered below that required for thermionic emission (but high enough to release barium) and an incident laser pulse was used to induce photoemission. These efforts resulted in the conclusion that thermionic cathodes cannot serve as photogated emitters in any commercially practical manner. Accordingly, ruggedizing traditional alkali-based photocathodes was attempted through use of compounds such as CsBr. These attempts have been largely unsuccessful because a shield to function as a membrane, the materials attempted must have finite thickness. This thickness has thus far been large enough to disrupt the photoemission process and negate the very effect it is designed to improve.
Accordingly, disclosed herein are embodiments of graphene and graphene composite shield enhanced photocathodes and methods for making the same. The graphene shield enhanced photocathodes provide long-lived photocathodes with high quantum efficiency (QE) at the longest possible wavelength. Although currently there are coatings used to enable semiconductor materials to photoemit with high QE, such materials last only a matter of hours in the vacuum environment of an electron gun. Herein a cathode is disclosed that has the long-life characteristic of metal emitters but the high QE of multi-alkali coatings. Currently available photocathodes emission efficiency degrades over time in a practical vacuum environment because of trace gases, Which contaminate and degrade the sensitive photocathode film. Embodiments of the presently disclosed photocathodes comprise graphene as a robust monolayer shield that protects the photosensitive film from damage, thus extending its lifetime and enabling the photocathode to be air-stable (i.e., capable of being handled after manufacture for limited time periods in air).
Certain disclosed embodiments of the graphene shield enhanced photocathode address a long-felt need for stabilizing a host of highly efficient, but chemically vulnerable, photosensitive films of photocathodes. In many accelerators, the photosensitive films launch photo-gated electron beams, often in extreme environments such as high field, high temperature, hostile and dynamic vacuum of a photoinjector. Photoemission-based electron injectors are widely used because of its current density being significantly higher than that of thermionic emission, providing high beam brightness with unsurpassed control over the spatial and temporal electron beam profile. One of the principle challenges for photoinjection is extending the lifetime of high efficiency photocathode operation. Embodiments of the disclosed graphene shield enhanced cathodes provide relatively stable high QE cathodes.
Embodiments of the graphene shield enhanced photocathodes effectively compensate for their short lifetime by providing a robust shield that is transparent to the incident photons and emitted electrons but isolates the photosensitive films from contaminating gases. Embodiments of the graphene shield enhanced photocathodes can be handled in hostile vacuum environments, perhaps even air, without degradation after manufacture, which vastly simplifies the procedures required for their use and directly translates to lower costs.
In one disclosed embodiment an electron emitting device comprises a photosensitive film and a graphene sheet positioned over the photosensitive film and in direct contact therewith. In another embodiment photocathode comprises a photosensitive film, a graphene sheet having a first surface and a second surface, a graphene support on a first portion of the first surface of the graphene sheet and configured to form a second portion of the first surface of the graphene sheet that is free of the graphene support, and wherein the second portion of the graphene sheet is positioned over the photosensitive film and is in direct contact therewith.
In another embodiment the photocathode substrate has a first surface, the first surface having a first portion and a second portion, a photosensitive film on the first portion of the first surface of the photocathode substrate and a graphene shield membrane directly contacting and completely covering the photosensitive film and connected to the second portion of the first surface of the photocathode substrate, forming a hermetic seal with the graphene shield membrane. The graphene shield membrane may further comprise a graphene support formed on a portion of a first surface of a graphene sheet. In certain embodiments the graphene support comprises a metal foil. The photocathode substrate may comprise molybdenum having an optical quality polish first surface. The graphene support may be configured to form a second portion of the first surface of the graphene sheet that is free of the graphene support.
In certain embodiments of the photocathode the graphene shield membrane comprises a plurality of graphene sheets such as 2 to 100 graphene sheets. In some embodiments the graphene shield membrane is connected to fixed supports in a vacuum environment and the cathode substrate and photosensitive film thereon are mounted to a linear actuator that is movable to place the photosensitive film in direct contact with the graphene shield membrane (while still under vacuum). The graphene shield membrane may be connected to the first surface of the cathode substrate and form a hermetic seal between the first surface of the cathode substrate and the graphene shield membrane.
In another embodiment a photocathode comprises a photosensitive film on a cathode substrate, a graphene shield membrane positioned to form a sealed chamber between the photosensitive film and the graphene shield membrane and capable of shielding the photosensitive film from reactive gas species. The graphene shield membrane may comprise a graphene composite having a graphene sheet with a first and a second surface and a scaffolding formed on the first and/or the second surface of the graphene sheet, the scaffolding capable of supporting the graphene sheet sufficient to form a vacuum sealed chamber between the graphene shield membrane and the photosensitive film.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.